Electrodynamic Linear Oscillating Motor

ABSTRACT

The invention relates to an electrodynamic linear oscillating motor characterised by high power densities in the magnet gap, magnetic return of the oscillating system to the centre position and a comparatively low weight of the oscillating system. The linear oscillating motor comprises a stator system, which is provided with at least one magnet ( 4 ) and an oscillating system which is supported such that it is able to move in the magnetic field of the stator. The oscillating system comprises at least one core ( 1 ) made of a magnetically soft material, e.g. a ferrite, and at least one driving coil ( 2, 3 ). The oscillating system is designed such that, if at least one driving coil ( 2,3 ) is de-energised, it is returned to the centre position by way of a reluctance force acting on the oscillating system. The electrodynamic linear motor in accordance with the present invention essentially combines the advantages of the known MC and MM linear motors, enabling electrodynamic conversion rates of up to 99% to be achieved. The linear oscillating motor is particularly suited as a drive for refrigeration and air-conditioning systems with low power ratings, as well as for pump, injection and shock absorber systems in automobile engineering.

The invention relates to an electrodynamic linear oscillating motor characterised by high power densities in the magnet gap, magnetic return of the oscillating system to the centre position and a comparatively low weight of the oscillating system. By way of this motor, it is possible to achieve electrodynamic conversion rates of up to 99%. The linear oscillating motor is particularly suited as a drive for refrigeration and air-conditioning systems with low power ratings, as well as for pump, injection and shock absorber systems in automobile engineering.

The compressors of low-power refrigeration and air-conditioning systems, as used in particular in household applications, are usually reciprocating compressors. For ecological reasons, rotary compressors, e.g. scroll compressors, are only used for equipment with drive outputs of several kilowatts.

Reciprocating compressors are usually driven by electric motors which produce a rotary motion. Crank mechanisms are required to convert this rotary motion into the translatory motion required for operation of the reciprocating compressor. It is here usual to use a Scotch yoke, with which the frictional forces between the piston and the cylinder liner are eliminated without the need for technically complex crosshead mechanisms, as in the case of other crank mechanisms. This achieves a high resistance to wear and a long service life of the drives, but as approx. 80% of the total friction occurs in the crank mechanism (in the Scotch yoke) and the motors are furthermore mainly rotary electric motors with low efficiency ratings between 50 and 70%, such systems usually achieve only a low efficiency of less than 50%.

At the same time, linear direct drives have been developed for reciprocating compressors for a number of years. For cost reasons, household refrigeration systems use above all electromagnetic linear motors (Maxwell motors). In addition, the field of linear drive technology for gas refrigeration machines to produce very low temperatures knows electrodynamic linear motors which can be distinguished functionally as systems with moving permanent magnets (MM) and systems with moving coil (MC).

In a Maxwell linear motor, based on the principle of minimisation of the magnetic field energy, a magnetically soft core is drawn into a coil when a voltage is applied to the latter. In accordance with this principle, springs or similar force elements are required to return the core when the voltage is reduced. Where Maxwell linear motors are used to drive reciprocating compressors, it is immanent to the system that a high proportion of the drive energy is lost in the springs.

By contrast, significantly higher levels of conversion efficiency between 60 and 90% (depending on the output class) can be achieved with electrodynamic linear motors. These motors are driven by Lorentz forces, whose amount and direction are dependent on the level and polarity of the operating voltage applied; they can thus be driven directly on AC voltages. Nevertheless, both MM motors and MC motors suffer certain design-related disadvantages.

MC motors/actuators offer the advantage that a large permanent magnet (GB 2 344 622 A and US 2006/208839 A1) or electromagnet (WO 98/50999 A1) can be used in the stator circuit, whereby high magnetic flux densities in the magnet gap and high drive forces can be realised. MC motors are thus well suited as drives for a low-speed (high-power) oscillating system, as required for the operation of reciprocating compressors. It is a disadvantage, however, that there is no magnetic position reset. Furthermore, moving power supply leads are required, though this disadvantage can be overcome to a large extent by way of a low-fatigue design.

From EP 1 158 547 A2, DE 10 2004 010 403 A1, WO 2008/046849 A1 and JP 2002031054 A, MM linear motors (or actuators) for use as drives for reciprocating compressors are known. Thanks to the reluctance force (principle of minimisation of the magnetic field energy), MM linear motors offer the advantage of system-immanent return of the oscillating system to its centre position, permitting the elimination of fatigue-prone mechanical reset systems, such as springs. Moving power supply leads are similarly not necessary. It is a disadvantage of MM linear motors, however, that the magnetic flux density in the magnet gap of the motor is relatively low, as the permanent magnet in the moving system must be designed to be as small and light as possible, in order not to impair the kinetics of the oscillating system. The consequently reduced drive forces could be compensated by higher speeds of the oscillating system, but high speeds of the oscillating system are unfavourable for applications as the drive of a reciprocating compressor.

FR 2 721 150 A1 discloses a multipolar system for the electrodynamic realisation of oscillation comprising a stator system and an oscillating system. The stator consists of a pole piece on which two magnets with opposing polarity are mounted. The oscillating system comprises two coils wound onto a pole piece supported such as to allow an oscillating motion.

As the moving pole piece (with constant cross-section) protrudes far beyond the two magnets of the stator system, practically no reluctance force acts on the oscillating system (when the coils are de-energised), even if the moving pole piece consists of a magnetically soft material, i.e. the oscillating system is not returned to its park/centre position. The system design is furthermore relatively complicated; it requires, in particular, two cost-intensive, radially magnetised permanent magnets. It is also a disadvantage that the design of the magnet circuit does not permit flux concentration in the magnet gap, as a result of which high force and power densities are impossible for such motors.

The purpose of the invention is to overcome the disadvantages of the prior art. It is in particular intended to reveal a linear oscillating motor characterised by high power densities in the magnet gap, magnetic return of the oscillating system to the centre position and a comparatively low weight of the oscillating system. The linear oscillating motor is to achieve high drive powers at low speeds of the oscillating system.

This objective of the present invention is achieved by the characteristic features of claim 1. Further advantageous embodiments and applications are to be derived from claims 2 to 14.

The starting point is an electrodynamic linear oscillating motor whose stator system is provided with at least one magnet, and whose oscillating system is supported such that it is able to move in the magnetic field of the stator.

In accordance with the invention, the oscillating system of the motor comprises at least one core of a magnetically soft material, e.g. a ferrite, and at least one coil. The oscillating system is designed such that, if at least one driving coil is de-energised, it is returned to the centre position by way of a reluctance force acting on the oscillating system.

It is intended that the magnet of the stator system of an electrodynamic linear oscillating motor in accordance with the present invention be realised as a large and strong magnet, as in the case of the currently known MC motors, so that a high magnetic flux density is achieved in the magnet gap of the motor. The motor can consequently be operated with high drive powers and low speeds of the oscillating system.

As it is immanent to the system of a linear motor in accordance with the present invention that the oscillating system is returned to its centre position (reluctance force), it is possible, as in the case of MC linear motors, to forego fatigue-prone mechanical reset systems (springs).

The principle applied enables the core of the oscillating system of a linear motor in accordance with the present invention to be designed such that it is significantly lighter than the permanent magnets of MM motors; the oscillating system comprising a core and at least one coil is heavier, however, than the plunger coils of MC motors.

An electrodynamic linear motor in accordance with the present invention thus essentially combines the advantages of the known MC and MM linear motors.

In a preferred embodiment, the stator system is designed as an annular-disk-shaped magnet magnetised in axial direction, with a ring-shaped pole disk made of a magnetically soft material located at each of the two end faces of said magnet. The internal and external diameters of the pole disks and those of the annular-disk-shaped magnet are identical. The oscillating system is concentric and supported such that it can move in the axial direction inside the stator system. It comprises a magnetically soft core, onto which two separate driving coils are wound such that said coils, when the oscillating system stands in its centre position, are each located in one of the magnet gaps formed by the pole disks in conjunction with the disk-shaped magnet. The orientation of the windings of the two driving coils is here chosen such that the Lorentz forces acting on the coils are accumulated when the motor is operated.

The core of the oscillating system is preferably cylindrical in form, with the external diameter of the oscillating system, which comprises the core and two coils wound onto said core, being smaller than the internal diameter of the pole disks.

Especially for use in refrigeration systems, and in line with the there usual rule of thumb “stroke=internal diameter”, the oscillating system of the motor can be arranged such that the stroke of the motor corresponds approximately to the internal diameter of the stator magnet. For the drives of compressors for household refrigeration units, the stroke amounts to approx. 10 to 20 mm.

Depending on the size/power of a linear motor in accordance with the present invention, an efficiency of up to 99% can be achieved. It is here the rule that the greater the power of the linear motor, the higher its efficiency.

A linear oscillating motor in accordance with the present invention is consequently very well suited both as a drive motor for the reciprocating compressors of air-conditioning and refrigeration systems and for single- or dual-piston linear compressors in gas refrigeration machines to produce very low temperatures.

Furthermore, the motor can be used advantageously as a drive motor for pumps transporting fuel, engine oil, cooling water or hydraulic fluid in automobile engineering. As it is possible to control the linear oscillating motor very quickly, it can also be used to control fuel injection in combustion engines. To this end, each mechanical valve is replaced by a valve controlled by way of a linear oscillating motor, and the camshaft control is replaced by fully electronic control.

As the electrodynamic drive principle is reversible, a linear oscillating motor in accordance with the present invention can also be operated as a linear generator, and is then particularly suited for electrical energy generation from drive systems with high power and short stroke, e.g. free-piston Stirling engines.

A further application, in which the linear oscillating motor acts simultaneously as drive and generator, involves use of the motor as damping for the independent suspension of motor vehicles. The damping force can here be controlled by way of the electrical load applied to the coils of the oscillating system. It can also be advantageous to provide the stator system with an electromagnet, as this offers an additional possibility to control the damping of the independent suspension by way of the strength of the magnetic field of the stator. The electrical energy generated, which is output in pulse form with strongly fluctuating voltage values, can be modified, e.g. by way of an inverter, and fed to the vehicle electrical system.

In the following, the invention is explained in greater detail by way of an embodiment and the following figures:

FIG. 1: Cross-section of a linear oscillating motor with the oscillating system in the centre position;

FIG. 2: Cross-section of a linear oscillating motor with the oscillating system displaced.

As to be seen in FIG. 1, the stator system of the linear oscillating motor comprises the annular-disk-shaped permanent magnet (4), which is magnetised in axial direction, and the first (5) and second (6) ring-shaped pole disks located at the two end faces of said magnet. The internal and external diameters of the pole disks (5, 6) correspond to those of the magnet (4).

The oscillating system is concentric and supported such that it can move in the axial direction inside the stator system and comprises the ferrite core (1), onto which the first (2) and second (3) driving coils are wound. While the oscillating system stands in its centre position, the first coil (2) is centred in the first magnet gap (7) and the second coil (3) correspondingly in the second magnet gap (8). The moving power supply leads for the coils (2, 3) are made of a fatigue-free material and are routed such that they are subject to only minimal bending.

To operate the linear oscillating motor, an AC voltage is applied to the driving coils (2, 3), producing an alternating electric current through the coils (2, 3). As the coils are arranged in the stator magnetic field, the coils through which the current flows are subjected to Lorentz forces, whose amount and direction are dependent on the level and polarity of the operating voltage applied. As the orientation of the windings of the two driving coils (2, 3) is such that the Lorentz forces acting on the coils (2, 3) are accumulated when the motor is operated, the applied AC voltage results in oscillation of the oscillating system at the same frequency as that of the AC voltage.

The linear oscillating motor operates with a frequency of 50 Hz, possesses a stroke of 10 mm, and delivers a mechanical power of 100 W at an efficiency of approx. 87%. It was surprisingly revealed that, during operation of the motor, the magnetic field of the stator remains practically unaffected by the motion of the oscillating system. It can be excluded, in particular, that the field lines follow the motion of the oscillating system, which would result in a reduction of the drive power.

LIST OF REFERENCES USED

-   1 Core of the oscillating system -   2 First driving coil -   3 Second driving coil -   4 Magnet of the stator system -   5 First pole disk -   6 Second pole disk -   7 First magnet gap -   8 Second magnet gap 

1. An electrodynamic linear oscillating motor with at least one magnet (4) in the stator system and an oscillating system which is supported such that it is able to move in the magnetic field of the stator, characterised in that the oscillating system of the motor comprises at least one core (1) made of a magnetically soft material and at least one driving coil (2, 3), and that the oscillating system is designed such that, if at least one driving coil (2, 3) is de-energised, it is returned to the centre position by way of a reluctance force acting on the oscillating system.
 2. A linear oscillating motor in accordance with claim 1, characterised in that the at least one magnet (4) of the stator system is a permanent magnet.
 3. A linear oscillating motor in accordance with claim 1, characterised in that the at least one magnet (4) of the stator system is an electromagnet.
 4. A linear oscillating motor in accordance with claim 1, characterised in that the core (1) of the oscillating system is made of a ferrite.
 5. A linear oscillating motor in accordance with claims 1 to 4, characterised in that the stator system comprises an annular-disk-shaped magnet (4) magnetised in axial direction, with a ring-shaped pole disk (5, 6) made of a magnetically soft material located at each of the two end faces of said magnet, the internal and external diameters of said pole disks being identical to those of the annular-disk-shaped magnet (4), and the oscillating system supported concentrically inside the stator system such that it can move in the axial direction comprises a magnetically soft core (1) onto which two separate driving coils (2, 3) are wound, wherein the two driving coils (2, 3), when the oscillating system stands in its centre position, are each located in one of the magnet gaps (7, 8) formed by the pole disks (5, 6) in conjunction with the disk-shaped magnet (4) and the orientation of the windings of the two driving coils (2, 3) is such that the Lorentz forces acting on the coils (2, 3) are accumulated when the motor is operated.
 6. A linear oscillating motor in accordance with claim 5, characterised in that the core (1) of the oscillating system is cylindrical in form and the external diameter of the oscillating system comprising the core (1) and the coils (2, 3) wound onto said core is smaller than the internal diameter of the magnet (4) of the stator system.
 7. A linear oscillating motor in accordance with claims 5 and 6, characterised in that the oscillating system of the motor is supported such that it can execute a stroke whose travel corresponds approximately to the internal diameter of the magnet (4) of the stator system.
 8. A linear oscillating motor in accordance with claims 5 to 7, characterised in that the oscillating system is able to execute a stroke of 10 to 20 mm.
 9. A linear oscillating motor in accordance with claims 1 to 8, characterised in that the oscillating system is designed for an operating frequency of approx. 50 Hz.
 10. Application of a linear oscillating motor in accordance with claim 1 as a drive motor for a reciprocating compressor for refrigeration and air-conditioning systems.
 11. Application of a linear oscillating motor in accordance with claim 1 as a drive motor for a pump for fuel, engine oil, cooling water or hydraulic fluid in automobile engineering.
 12. Application of a linear oscillating motor in accordance with claim 1 for the injection of fuel in combustion engines.
 13. Application of a linear oscillating motor in accordance with claim 1 as a drive motor for single- or dual-piston linear compressors in gas refrigeration machines to produce very low temperatures.
 14. Application of a linear oscillating motor in accordance with claim 1 as a generator for electrical energy generation from drive systems with high power and short stroke.
 15. Application of a linear oscillating motor in accordance with claim 1 as a current generator designed as an electrodynamic damping system for the independent suspension of motor vehicles. 